[foreign-jextract] Integrated: 8254156: Simplify ABI classification logic

[Maurizio Cimadamore]

This small patch fixes SourceConstantHelper to use the more direct classification logic.

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Commit messages:
 - Fix SourceConstantHelper

Changes: https://git.openjdk.java.net/panama-foreign/pull/372/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=372&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254156
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/372.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/372/head:pull/372

PR: https://git.openjdk.java.net/panama-foreign/pull/372